Bonding wire for semiconductor devices

ABSTRACT

A semiconductor-device bonding wire includes a core member formed of an electrically-conductive metal, and a skin layer mainly composed of a face-centered cubic metal different from the core member and formed thereon. An orientation ratio of &lt;111&gt; orientations in crystalline orientations &lt;hkl&gt; in a wire lengthwise direction at a crystal face of a surface of the skin layer is greater than or equal to 50%, and the &lt;111&gt; orientations have an angular difference relative to the wire lengthwise direction, the angular difference being within 15 degrees.

CROSS-REFERENCE TO PRIOR APPLICATION

This is the U.S. National Phase Application under 35 U.S.C. §371 ofInternational Patent Application No. PCT/JP2008/071899 filed Dec. 2,2008, which claims the benefit of Japanese Patent Application No.2007-312238, filed Dec. 3, 2007, and Japanese Patent Application No.2008-295178, filed Nov. 19, 2008, all of which are incorporated byreference herein. The International Application was published inJapanese on Jun. 11, 2009 as WO2009/072498 A1 under PCT Article 21(2).

TECHNICAL FIELD

The present invention relates to a semiconductor-device bonding wire forconnecting an electrode on a semiconductor element and an externalterminal together.

BACKGROUND ART

Currently, thin wires (bonding wires) having a wire diameter of 20 to 50μm or so are popularly used as bonding wires for connecting an electrodeon a semiconductor element and an external terminal together which is awiring of a circuit wiring board (a lead frame, a substrate, a tape orthe like). A thermal compressive bonding technique with the aid ofultrasound is generally applied to bond bonding wires, and ageneral-purpose bonding device, and a capillary jig which allows abonding wire to pass through the interior thereof for connection areused. A leading end of a bonding wire is heated and melted by arc heatinputting, a ball is formed by surface tension, and then the ball iscompressively bonded on an electrode of a semiconductor element heatedwithin a range from 150 to 300° C. beforehand. Thereafter, the bondingwire is directly bonded to an external lead by ultrasound compressivebonding.

Recently, technologies related to the structure, material and connectionfor the semiconductor packaging technologies are rapidly diversified,and for example, in a packaging structure technology, in addition tocurrently-used QFP (Quad Flat Packaging) using a lead frame, newconfigurations, such as BGA (Ball Grid Array) using a substrate, apolyimide tape or the like and CSP (Chip Scale Packaging) arepractically used, and a bonding wire which has improved loopcharacteristic, bonding property, mass productivity, usability and thelike becomes requisite.

Introduction of a fine pitch technique that a space between adjacentbonding wires becomes narrow has progressed. Thinning, improvement ofstrength, loop controllability, and bonding property become requisitefor bonding wires in accordance with the introduction of the fine pitchtechnique. A loop shape becomes complex together with the density growthof the semiconductor packaging technologies. A loop height and a wirelength (span) of a bonding wire are barometers for classification of theloop shape. In most-recent semiconductor devices, contradictory loopshapes, such as a high loop and a low loop, and, a short span and a longspan, are mixed within the interior of a single package. In order torealize such contradictory loop shapes with a bonding wire of one kind,a strict designing of a material of a bonding wire is essential.

So far, 4N-group gold having a high purity (purity >99.99% by weight) isused as a material of a bonding wire. In order to improve strength and abonding property, a tiny amount of alloy elements are prepared.Recently, a gold alloy wire with a purity of 2N (purity >99%) that anadditive element concentration is increased to less than or equal to 1%becomes into practical use in order to improve the reliability of abonded part. The strength can be improved and the reliability can becontrolled by adjusting the kind and the concentration of an alloyelement added to gold. Conversely, alloying may cause deterioration of abonding property and increment of an electrical resistance, so that itis difficult to comprehensively satisfy various characteristicsrequisite for the bonding wires.

Moreover, because gold is expensive, other kinds of metals which have alow material cost are desired, and bonding wires which have a lowmaterial cost, have a good electrical conductivity and are made ofcopper are created. According to the copper bonding wires, however, abonding strength is reduced due to oxidization of a wire surface, and awire surface is likely to be corroded when encapsulated in a resin.These are the reasons that practical usage of the copper bonding wiresis not accelerated.

All bonding wires in practical use so far have a monolayer structure.Even if materials, such as gold and copper, are changed, alloy elementsare uniformly contained in a bonding wire, and a wire monolayerstructure is employed as viewed from a cross section of a bonding wire.A thin native oxide film, an organic film for protecting a surface andthe like may be formed on a wire surface of a bonding wire in somecases, these are limited in an extremely-thin area (up to several atomiclayer level) in an outermost surface.

In order to meet various needs requisite for the bonding wires, abonding wire with a multilayer structure in which a wire surface iscoated with another metal is proposed.

As a technique of suppressing any oxidization of a surface of a copperbonding wire, patent literature 1 discloses a bonding wire in whichcopper is covered with a noble metal or a corrosion-resistant metal,such as gold, silver, platinum, palladium, nickel, cobalt, chrome,titanium, and the like. Moreover, from the standpoint of a ballformability and suppression of deterioration of a plating solution,patent literature 2 discloses a bonding wire so structured as to have acore member mainly composed of copper, a dissimilar metal layer formedon the core member and made of a metal other than copper, and a coatinglayer formed on the dissimilar metal layer and made of anoxidization-resistant metal having a higher melting point than copper.Patent literature 3 discloses a bonding wire comprising a core membermainly composed of copper, and an outer skin layer which contains ametal, having either one of or both of a constituent and a texturedifferent from the core member, and copper, and which is a thin filmhaving a thickness of 0.001 to 0.02 μm.

Various gold bonding wires with a multilayer structure have beenproposed. For example, patent literature 4 discloses a bonding wirecomprising a core wire formed of highly-pure Au or Au alloy, and acoating material coating an outer surface of the core wire and formed ofa highly-pure Pd or Pd alloy. Patent literature 5 discloses a bondingwire comprising a core wire formed of highly-pure Au or Au alloy, and acoating material coating an outer surface of the core wire and formed ofhighly-pure Pt or Pt alloy. Patent literature 6 discloses a bonding wirecomprising a core wire formed of highly-pure Au or Au alloy, and acoating material coating an outer surface of the core wire and formed ofhighly-pure Ag or Ag alloy.

It is desirable to cope with the most advanced fine pitch technique andhigh-density packaging technique like three-dimensional wiring bysatisfying comprehensive characteristics such that loop control isstable in a bonding process, a bonding property is improved, deformationof a bonding wire at the time of resin encapsulation is suppressed, anda long-term reliability of a bonded part is accomplished as wirecharacteristics of a bonding wire of mass-production.

Regarding ball bonding, it is important to form a ball with a goodsphericity at the time of ball formation, and to obtain a sufficientbonding strength between the ball and an electrode. Moreover, in orderto cope with lowering of a bonding temperature and thinning of a bondingwire, a bonding strength, a tensile strength and the like are requisiteat a part where a bonding wire is subjected to wedge bonding to a wiringon a circuit wiring board.

A surface nature of a wire affects a use performance, and for example,merely creation of a flaw and that of scraping make mass-productiondifficult. Adjoining bonding wires may be electrically short-circuiteddue to scraping, and a flaw makes the quality and reliability of abonding wire, such as a manufacturing yield of a bonding wire and wiredeformation at the time of resin encapsulation, deteriorated. Moreover,practical use cannot be accomplished if strict requisitions such that afailure occurrence rate of a semiconductor manufacturing process ismanaged in a ppm order are not satisfied by further pursuing a stabilityof loop-shape control and by further improving a bonding property at alow temperature.

Such bonding wires with a multilayer structure for semiconductors havelarge anticipation for practical use, but have not become in practicaluse. Reforming of a surface, a high added value, and the like by themultilayer structure is anticipated, but in contrast, it is necessary tocomprehensively satisfy a productivity of a bonding wire, a qualitythereof, a yield in a bonding process, performance stability, and along-term reliability at the time of semiconductor usage.

-   Patent Literature 1: JPS62-97360A-   Patent Literature 2: JP2004-64033A-   Patent Literature 3: JP2007-12776A-   Patent Literature 4: JPH04-79236A-   Patent Literature 5: JPH04-79240A-   Patent Literature 6: JPH04-79242A

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

According to the conventional bonding wires with a monolayer structure(hereinafter, “monolayer wire”), addition of an alloy element iseffective to improve a tensile strength, a strength of a bonded part, areliability thereof, and the like, but it is thought that there is alimit in improvement of the characteristics. According to the bondingwires with a multilayer structure (hereinafter, “multilayer wire”), itis anticipated that the characteristics are further improved incomparison with the monolayer wires to improve an added value. Regardingthe multilayer wires bringing a high functionality, a wire surface canbe coated with a noble metal or an oxidation-resistant metal to suppressany oxidization of a surface of a copper bonding wire. For gold bondingwires, it is anticipated to accomplish an effect of reducing resin sweepby coating a wire surface with a metal or an alloy having a highstrength.

However, according to the study of the inventors of the presentinvention in consideration of density growth, miniaturization, andthinning of the semiconductor packaging technology, it becomes clearthat the multilayer wires have lots of following practical problems.

Regarding the multilayer wires, a flaw, scraping and the like are likelyto be formed on a surface of a wire because of drawing in a wiremanufacturing process and complex loop control in a wire bonding processwhen a multilayer wire is used as a wire final product or used forconnecting a semiconductor element. For example, a flaw of a wiresurface may create a minute groove in a submicron order, and as afailure example of scraping, shaved-piece-like scraping is created inthe lengthwise direction of a multilayer wire, and the length of suchscraping becomes up to several hundred μm. The flaw and scraping on asurface make a loop shape unstable, a multilayer wire is damaged so thatthe strength thereof is reduced, and if a scraped piece contactsadjoining multilayer wires, it causes short-circuiting, resulting inpractical problems.

The thinner a wire diameter is, the more the frequency of occurrence offailures relating to a flaw and scraping on a surface increases, and itis not appropriate for fine pitch connection, and such a frequency alsoincreases as loop control becomes complex such that a high loop and alow loop are mixed, and it is difficult to apply the multilayer wires tothree-dimensional connection. The frequency of occurrence of a surfaceflaw is likely to increase when a low loop is formed. If such failure isnot reduced, the practical application of the multilayer wires is stilllimited.

In addition to such direct failures, indirect failures due to a flaw andscraping on a wire surface, or reduction of the yield are alsoconcerned. For example, a flaw and scraping once created in the middlestage of manufacturing of multilayer wires cannot be detected from afinal product, but make the thickness of a skin layer uneven, and make aloop shape unstable as an internal crack remains. Moreover, a flaw andscraping created in the back of a loop are not likely to be detectedthrough a loop external appearance test using an optical microscope in amass-production stage. Such failures result in reduction of amanufacturing yield even though direct causal relationships with a flawand scraping are not acknowledged easily.

The frequency of occurrence of a surface flaw and scraping andphenomenon thereof vary depending on a material of a skin layer, butthere is no sufficient countermeasure so far. The occurrence frequencyin the case of the multilayer wires often increases in comparison withthe case of the monolayer wires, and it is thought that this is because,in the multilayer wires, loads, such as stress and strain, relative to askin layer increase in a process of forming a loop and because a processcondition differs in wire manufacturing processes.

When a loop is formed using a multilayer wire, the linearity of the loopmay decrease, and defects, such as fall-down, grow-down and bending ofthe multilayer wire may occur. As the linearity of the loop decreases, amanufacturing yield also decreases.

Typical defects of a ball bonded part of the multilayer wires are apetal-like deformation phenomenon and a misalignment phenomenon. Thepetal-like deformation phenomenon is that roundness is deteriorated asthe proximity of the outer circumference of a ball bonded part isdeformed in a petal-like shape concavely and convexly, so that a ball isout of an area of an electrode when bonded on a small electrode, and isa cause of failure like reduction of a bonding strength. Themisalignment phenomenon is that a ball formed at a leading end of a wireis unsymmetrically formed relative to a wire axis and the wire is formedin a shape like a golf club, and when a misaligned ball is bonded infine pitch connection, such a ball may contact an adjoining ball,resulting in a short-circuiting. The occurrence frequencies of thepetal-like deformation phenomenon and the misalignment phenomenon in thecase of the multilayer wires tend to increase in comparison with thecase of the monolayer wires, resulting in reduction of the productivity,so that it is necessary to tighten management criteria for the wirebonding process.

It is anticipated that the multilayer copper wires have a better effectof delaying oxidization than the monolayer copper wires, but such aneffect largely varies depending on a texture, a structure, a thickness,and the like at a skin layer or at the proximity of a wire surface. Itis important to make the structure of the multilayer copper wiresappropriate. In order to ensure the same workability as that of goldwires, for example, it is necessary to ensure that a wedge bondingproperty, a loop shape and the like are not deteriorated even afterwires are stored for two months or so in a normal atmosphere. Thisrequires improvement of a life several ten times in comparison with thestorage life of the monolayer copper wires, so that a considerably hardcondition is required for materials mainly composed of copper.

It is an object of the present invention to provide asemiconductor-device bonding wire which can overcome the problems of theconventional technologies, suppress any formation of flaws and scrapingon a wire surface, and accomplish improvement of characteristics, suchas stabilization of a loop shape and good ball formation, in addition toconventional basic characteristics.

Means for Solving the Problem

In order to overcome problems, such as flaws and scraping on a wiresurface, the inventors of the present invention keenly studied bondingwires with a multilayer structure, and found that it is effective tocontrol a texture of a specific skin layer.

The present invention has been made based on the foregoing knowledge,and employs following structures.

A bonding wire for semiconductor devices according to the first aspectof the present invention comprises: a core member formed of anelectrically-conductive metal; and a skin layer formed on the coremember and mainly composed of a different metal from the core member,and wherein the metal of the skin layer is a face-centered cubic metal,and a percentage of <111> orientation in crystalline orientations <hkl>in a lengthwise direction at a crystal face of a surface of the skinlayer is greater than or equal to 50%.

According to the semiconductor-device bonding wire with the foregoingstructure according to the second aspect of the present invention,wherein a total percentage of <111> orientation and <100> orientation inthe crystalline orientations <hkl> in the lengthwise direction at thecrystal face of the surface of the skin layer is greater than or equalto 60%.

According to the semiconductor-device bonding wire with the foregoingstructure according to the third aspect of the present invention,wherein a total percentage of <111> orientation and <100> orientation inthe crystalline orientations <hkl> in the lengthwise direction at acrystal face of a cross section of the core member is greater than orequal to 15%.

According to the semiconductor-device bonding wire with the foregoingstructure according to the fourth aspect of the present invention,wherein a percentage of <111> orientation and <100> orientation in thecrystalline orientations <hkl> in the wire lengthwise direction at thecrystal face of the cross section of the core member is greater than orequal to 30%.

According to the semiconductor-device bonding wire with the foregoingstructure according to the fifth aspect of the present invention,wherein a ratio of an average size of a crystal grain at the surface ofthe skin layer in the lengthwise direction relative to an average sizeof the crystal grain at the surface of the skin layer in thecircumferential direction is greater than or equal to three.

According to the semiconductor-device bonding wire with the foregoingstructure according to the sixth aspect of the present invention,wherein a percentage of an area of crystal grains where crystallineorientation in the wire lengthwise direction at the surface of the skinlayer is <111> is greater than or equal to 30% relative to a total areaof a wire surface.

According to the semiconductor-device bonding wire with the foregoingstructure according to the seventh aspect of the present invention,wherein a main constituent of the skin layer is at least one kind offollowings: Pd; Pt; Ru; and Ag.

According to the semiconductor-device bonding wire with the foregoingstructure according to the eighth aspect of the present invention,wherein a main constituent of the core member is at least one kind offollowings: Cu; and Au.

The semiconductor-device bonding wire with the foregoing structureaccording to the ninth aspect of the present invention furthercomprising an intermediate metal layer formed between the skin layer andthe core member and composed of a different constituent from the mainconstituent of the skin layer and the main constituent of the coremember.

According to the semiconductor-device bonding wire with the foregoingstructure according to the tenth aspect of the present invention,wherein a thickness of the skin layer is within a range from 0.005 to0.2 μm.

The semiconductor-device bonding wire with the foregoing structureaccording to the eleventh aspect of the present invention furthercomprising a diffusion layer formed between the skin layer and the coremember and having a concentration gradient.

According to the semiconductor-device bonding wire with the foregoingstructure according to the twelfth aspect of the present invention,wherein the main constituent of the core member is Cu and the coremember contains greater than or equal to one kind of followings: B; Pd;Bi; P; and Zr at a concentration within a range from 5 to 300 ppm.

According to the semiconductor-device bonding wire with the foregoingstructure according to the thirteenth aspect of the present invention,wherein the main constituent of the core member is Cu and the coremember contains Pd at a concentration within a range from 5 to 10000ppm, and the main constituent of the skin layer is Pd or Ag.

According to the semiconductor-device bonding wire with the foregoingstructure according to the fourteenth aspect of the present invention,wherein the main constituent of the core member is Au and the coremember contains greater than or equal to one kind of followings: Be; Ca;Ni; Pd and Pt at a concentration within a range from 5 to 8000 ppm.

Effect of the Invention

The semiconductor-device bonding wire of the present inventionsuppresses any formation of flaws and scraping on a wire surface,thereby improving a surface nature. Moreover, the linearity of a loopand the stability of loop heights are improved. Furthermore,stabilization of a bonding shape of the semiconductor-device bondingwire is promoted. As a result, there is provided the highly-functionalsemiconductor-device bonding wire which can cope with latestsemiconductor packaging technologies, such as thinning, a fine pitch, along span, and three-dimensional packaging.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an EBSP measurement result of a bonding wire (wirediameter: 25 μam) with a multilayer structure (an area oriented to <111>orientation is colored. A crystalline interface is indicated by a line).

BEST MODE FOR CARRYING OUT THE INVENTION

Regarding semiconductor-device bonding wires (hereinafter, “bondingwires”), one comprising a core member formed of anelectrically-conductive metal and a skin layer formed on the core memberand mainly composed of a face-centered cubic metal different from thecore member were studied, and it became clear that a wedge bondingproperty can be improved if an electrically-conductive metal arecontained in the proximity of a surface of a bonding wire, but formationof a flaw, scraping and the like on a wire surface by wire drawing in awire manufacturing process and by complex loop control in a wire bondingprocess becomes a problem, and the stability of a loop shape isinsufficient.

Accordingly, the inventors of the present invention studied bondingwires with a multilayer structure which can cope with new needs likestrict loop control in fine pitch connection and in three-dimensionalconnection, and which can accomplish improvement of a yield in drawingof a thin wire, and found that controlling of a texture of a specificskin layer is effective. In particular, the inventors of the presentinvention focused on a relation, which was hardly known so far, betweena texture of a surface of multilayer wires and the usability of bondingwires, and verified that the workability, the bonding property, the loopcontrollability and the like can be comprehensively improved bycontrolling a specific crystalline orientation. Moreover, the inventorsof the present invention found out that controlling of a combination ofthe texture of the skin layer and that of the core member is furthereffective.

That is, it is necessary that such a bonding wire must comprise a coremember formed of an electrically-conductive metal and a skin layerformed on the core member and mainly composed of a face-centered cubicmetal different from the core member, wherein greater than or equal to50% of crystalline orientations <hkl> in the lengthwise direction at acrystal face of a surface of the skin layer is <111>. According to sucha bonding wire, a superior effect of suppressing any formation of flawsand scraping on a wire surface by wire drawing in a wire manufacturingprocess and by complex loop control in a wire bonding process can beaccomplished.

If a constituent of the skin layer is a face-centered cubic metal, thereis no yield drop at the time of working, and the workability is good,resulting in facilitation of adaptation to complex working, bending andthe like in wire drawing and loop controlling.

As greater than or equal to 50% of the crystalline orientations <hkl> inthe lengthwise direction of a bonding wire is <111>, a set ofcharacteristics, such as the surface strength of the skin layer, aworkability, a bending tolerability and the like which are difficult toaccomplish simultaneously can be improved, and as a result, anyformation of flaws and scraping on a wire surface can be suppressed. The<111> orientation of the face-centered cubic metal is a most densedirection, and if such <111> orientations are collected in the skinlayer, the mechanical strength of the surface is likely to increasemore, and for example, the strength is enhanced, the tolerabilityagainst elastic deformation and the tolerability against plasticdeformation become high, and the toughness can be also enhanced. If thepercentage of the <111> orientations in the crystalline orientations isgreater than or equal to 50%, a sufficient effect of suppressing anyformation of flaws and scraping on a wire surface can be accomplished.Preferably, if the percentage of the <111> orientations is greater thanor equal to 60%, an effect of suppressing any formation of scraping isfurther improved, and formation of any scraping and flaw can be reducedat a long span in which a wire length is greater than or equal to 5 mm.More preferably, if it is greater than or equal to 70%, an effect ofsuppressing any formation of flaws is further improved, and formation ofany flaw and scraping can be suppressed even in the case of formation ofa low loop in which a loop height is, for example, less than or equal to65 μm, and formation of a loop can be stabilized.

According to the multilayer wires, because the skin layer and the coremember are formed of different constituents, it is relatively easy toseparately control a texture of the skin layer covering a wire surface.Controlling of the surface texture also brings a good effect ofcharacteristic improvement. It is thus different from texturecontrolling in the case of the conventional monolayer wires. Accordingto the monolayer wires, it is possible to manage a texture of a wholewire and a crystalline orientation, but it is difficult to control atexture of only proximity of the surface separately from the interior ofthe wire. Accordingly, a unique scheme for the case of the multilayerwires is required for controlling a texture of the skin layer of themultilayer wires, and management scheme of a texture and a crystallineorientation in a wire cross section in the case of the monolayer wirescannot be applied.

If the total percentage of <111> and <100> orientations in thecrystalline orientations <hkl> in the lengthwise direction at thecrystal face of the surface of the skin layer is greater than or equalto 60%, dispersion in loop heights can be reduced, and controlling ofthe stability of a loop at a fast operation speed becomes easy. In anormal wire connecting process, a bonding wire passing through anaperture of a capillary is subjected to complex motion, such aspulling-out and pulling-back. This is a motion that the bonding wire ispulled out and pulled back at a very fast speed in an order of severalten milliseconds. Specific effects of individual <111> and <100>orientations and a correlation therebetween are not clear yet, but it isthought that a loop height is stabilized as a sliding resistance betweenthe bonding wire and the capillary is reduced. In other words, in orderto stabilize the slidability and the loop height, it is effective tosuppress any crystalline orientations other than <111> and <100>orientations. If the total percentage of <111> and <100> orientations inthe skin layer is greater than or equal to 60%, a good effect ofstabilizing a loop height at a normal span in which a wire length isless than or equal to 3 mm can be accomplished. Preferably, if it isgreater than or equal to 80%, a good effect of stabilizing a loop heightcan be accomplished at a long span in which a wire length is greaterthan or equal to 5 mm. Moreover, as an effect accomplished by increasingthe percentage of the <111> and <100> orientations, unevenness of a filmthickness in a working after formation of the film and in a thermaltreatment process can be suppressed, making the thickness of the skinlayer uniform.

If the total percentage of the <111> and <100> orientations in thecrystalline orientations <hkl> in the wire lengthwise direction at across section of the core member is greater than or equal to 15%, it ispossible to suppress any failures that a ball bonded part is abnormallydeformed and is out of a true circle. Such abnormal deformation maydirectly cause electrical short-circuiting with adjoining electrode, andis one of failures most concerned in ball bonding. As a rough indicationfor a criterion for checking abnormal deformation, an elliptical shapethat a ratio between a longer diameter size of a ball bonded part and ashorter diameter size thereof is greater than or equal to 1.3 times isdetermined as a failure. Even if such a failure occurs in an unexpectedfashion at a low occurrence rate, it deteriorates the productivity ofthe bonding wire. Preferably, if the total percentage of <111> and <100>orientations is greater than or equal to 30%, a tiny petal-likedeformation failure that the proximity of the outer circumference of aball bonded part deform concavely and convexly can be suppressed, theball bonded part becomes close to a true circle and is stabilized. Goodroundness is advantageous for reduction of a bonding area, so thatmanufacturing management in a bonding process is facilitated, and theproductivity in fine pitch connection is also improved. It is verifiedthat the solidification structure of a ball greatly reflects the textureof the core member, and increment of the percentage of <111> and <100>orientations in the crystalline orientations <hkl> of the core member iseffective. It is verified that such controlling of crystallineorientations of the core member does not bring a sufficient effect inthe case of the monolayer wires, but brings a good effect in the case ofthe multilayer wires. The reason why is not completely clarified yet,but it is thought that the texture of the core member significantlyaffects the texture of a ball because the skin layer is melted first,and the core member is then melted step by step in ball melting of themultilayer wires. It is verified that better action and effect can beaccomplished when a ball is in a normal ball size. For example, when aball in a normal size that the ratio between an initial ball diameterand a wire diameter is 1.9 to 2.2 is bonded, anisotropy and a shapefailure like petal-like deformation at a ball bonded part can besuppressed, thereby improving the roundness. The inventors of thepresent invention studied deformation behavior of a ball by compressivedeformation and application of ultrasound, and verified that acorrelation between a ball bonding shape and the texture of the skinlayer is little, but the texture of the core member dominantly affects.If the percentage of <111> and <100> orientations in the core member isless than 15%, the frequency of occurrence of abnormal deformation atthe time of ball bonding increases, and if it is less than 30%, thefrequency of occurrence of deformation in a petal-like shape and anelliptical shape increases, resulting in a failure occasionally. Theeffect of the texture of the wire on ball deformation is remarkable inthe case of the multilayer wires, and is different from the effect ofthe texture of the monolayer wires in many cases. Preferably, if thetotal percentage of <111> and <100> orientations in the core member isgreater than or equal to 50%, a ball with a small diameter can have astable bonding shape. For example, when a ball with a small diameterthat the ratio between an initial ball diameter and a wire diameter is1.5 to 1.7 is subjected to bonding, if the roundness of a ball bondedpart is improved, a good ball bonding shape can be acquired even in thecase of a fine pitch connection with an electrode clearance less than orequal to 40 μm. There is no specific uppermost limit for the percentageof the <111> and <100> orientations in the core member, but if it isless than or equal to 85%, there is an advantage that controlling at thetime of manufacturing becomes relatively easy.

A synergistic effect by combining the texture of the core member and thetexture of the skin layer is anticipated which simultaneously improvescontrolling of a loop shape and stabilization of ball deformation. Thatis, it is desirable that a bonding wire with a multilayer structureshould have a composition in which greater than or equal to 50% of thecrystalline orientations <hkl> in the lengthwise direction at a crystalface of the surface of the skin layer is <111> and the percentage of<111> and <100> orientations in the crystalline orientations <hkl> inthe wire lengthwise direction at a cross section of the core member isgreater than or equal to 40%. Accordingly, the comprehensivecharacteristics of the bonding wire can be improved in three-dimensionalpackaging, such as stacked-chip connection that a plurality of chips arestacked together, and multi-tier bonding that the loop heights ofadjoining bonding wires largely vary within a range from 60 to 500 μmwhich is recently applied to BGA and CSP.

The explanation has been given of the action and the effect of thepercentage of specific orientations with reference to measurablecrystalline orientations. If thinning advances in near future for copingwith accomplishment of a fine pitch, the degree of influence by asurface increases, so that it becomes possible to figure out the effectof practical use if the effect of crystalline orientation is organizedwith reference to the surface of a bonding wire.

More specifically, it is desirable that a bonding wire with a multilayerstructure should have a composition in which greater than or equal to50% of the crystalline orientations <hkl> in the lengthwise direction atthe crystal face of the surface of the skin layer is <111> and an areaof crystal grains where the crystalline orientations in the wirelengthwise direction at the surface of the skin layer is <111> isgreater than or equal to 30% as a ratio relative to the total area ofthe wire surface. Accordingly, an effect of stabilizing a loop shape isenhanced, and in particular, a loop characteristic is stabilized even inthe case of a bonding wire thinned so as to have a diameter less than orequal to 22 μm, and it is effective for reduction of any formation offlaws and scraping. According to a thin bonding wire having a wirediameter less than or equal to 22 μm, due to increment of strain by wiredrawing or the like, an area of crystalline orientations which aredifficult to measure increases, so that an area where a loopcharacteristic cannot be precisely grasped only through the percentageof the <111> orientations in the measurable crystalline orientations islikely to increase. Therefore, if the area of crystal grains having the<111> orientation in the surface of the skin layer is set to be anappropriate ratio (appropriate area ratio) in a ratio (area ratio)relative to the total area of the wire surface, good characteristics canbe acquired even in the case of thin wires. The reason why the arearatio is set to be greater than or equal to 30% is because any formationof flaws and scraping cannot be suppressed in some case when fine pitchconnection is carried out using a bonding wire with a wire diameter lessthan or equal to 22 μm if the area ratio is less than 30% even thoughgreater than or equal to 50% of the crystalline orientations is <111>.Preferably, if the area ratio is greater than or equal to 40%, anyformation of flaws and scraping even when a thin wire with a wirediameter of less than or equal to 18 μm is connected to form a loop.More preferably, if the area ratio is greater than or equal to 50%, aneffect of suppressing any formation of flaws and scraping is furtherenhanced even in the case of thin wires with a diameter less than orequal to 18 μm, and it is advantages for fine pitch connection with adiameter less than or equal to 40 μm.

If a bonding wire with a multilayer structure has a composition in whichgreater than or equal to 50% of crystalline orientations <hkl> in thelengthwise direction at a crystal face of a surface of the skin layer is<111> and a ratio (an aspect ratio of crystal grain diameters) of anaverage size of a crystal grain in the surface of the skin layer in thelengthwise direction relative to an average size thereof in thecircumferential direction is greater than or equal to three, thelinearity of the bonding wire having undergone loop formation can beimproved. When the bonding wire is pushed out from the aperture in theleading end of the capillary and is pushed back therein to form a loop,a curl failure as the bonding wire falls down and is bent because offriction with the internal wall of the capillary, and a bonding failureas the bonding wire grows down occur, resulting in reduction of a yield.In order to suppress such failures and to improve the loop linearity,the inventors of the present invention found that it is effective if theaspect ratio of the crystal grain at the surface of the skin layer isincreased. If the aspect ratio is increased, crystal grains long in thewire lengthwise direction form fibrous texture, and it is advantageousfor reducing a residual strain in the bonding wire at the time of loopformation and deformation dispersion thereof. If the aspect ratio isgreater than or equal to three, an effect of improving the looplinearity can be sufficiently accomplished. Preferably, if the aspectratio is greater than or equal to five, a good loop linearity can beacquired at a long span in which a wire diameter is less than or equalto 25 μm and a wire length is longer than or equal to 5 mm. Morepreferably, if the aspect ratio is greater than or equal to ten, aneffect of improving the loop linearity at a super long span in which awire length is greater than or equal to 7 mm can be enhanced.

The face-centered cubic metal which is a main constituent of the skinlayer is a metal different from the electrically-conductive metal whichis a main constituent of the core member, and it is desirable that sucha metal should have an effect of improving the bonding property of thebonding wire and be effective for suppressing any oxidization of thebonding wire. More specifically, Pd, Pt, Ru, Rh, and Ag are candidatesof such a metal, and at least one kind of followings: Pd; Pt; Ru; and Agis desirable from the standpoint of practical utility and costperformance. Note that a main constituent in the embodiment means anelement having a concentration greater than or equal to 50 mol %. Pd hasadvantages that adhesiveness to an encapsulation resin and bondingproperty to an electrode are sufficient and quality management is easy.Pt makes stabilization of a ball shape relatively easy. Ru is a hardmetal, facilitates formation of a dense film, and has a relativelyinexpensive material cost. Rh has a good oxidization resistance and thelike, but has an expensive material cost, so that a future examinationthereof like thinning is anticipated. Ag is a soft material, makessuppression of any formation of flaws, formed by drawing of a wireformed with the skin layer beforehand, relatively easy, and has aninexpensive material cost, so that it is appropriate for cost-emphasizedsemiconductors.

That is, it is preferable that the skin layer should be a pure metalmainly composed of at least one kind of followings: Pd; Pt; and Ru, or,should be an alloy mainly composed of such an electrically-conductivemetal. If the skin layer is a pure metal, it is advantageous thatimprovement of oxidization resistance and bonding property is relativelyeasy, and if the skin layer is an alloy, there is an advantage ofincreasing the tensile strength and the elastic modulus, resulting insuppression of any wire deformation at the time of resin encapsulation.Note that a pure metal in the embodiment means that a layer having aconcentration greater than or equal to 99 mol % is contained in a partof the skin layer, or the average concentration of the skin layer otherthan a diffusion layer is greater than or equal to 80 mol %. An alloymeans one containing at least one kind of followings: Pd; Pt; and Ru ata concentration greater than or equal to 50 mol %.

Cu, Au, and Ag are candidates for the electrically-conductive metal ofthe core member, and from the standpoint of practical utility, it isdesirable that the core member should be mainly composed of at least onekind of followings: Cu; and Au. Cu has an inexpensive material cost, hasa good electrical conductivity, and relatively-good handleability suchthat ball formation is easy if sprayed with a shield gas at the time ofball formation. Au has a high oxidization resistance, does not require ashield gas at the time of ball formation, has a good deformationbehavior at the time of bonding, and is easy to ensure the bondingproperty. Ag has a good conductivity, but is slightly difficult to dowire drawing, so that it is necessary to make the manufacturingtechnique appropriate. Conversely, Cu and Au have an advantage thatthose are time-proven materials for the monolayer bonding wires.

If the core member is formed of an alloy mainly composed of anelectrically-conductive metal, there is an advantage in some cases forthinning or improvement of the bonding reliability because the wirestrength is increased. In the case of the Cu alloy, if such an alloycontains at least one kind of followings: B; Pd; Bi; and P within arange from 5 to 300 ppm, the tensile strength of the bonding wire andthe elastic modulus thereof increase, resulting in accomplishment of aneffect of improving the linearity at a long span up to 5-mm or so. Inorder to enhance such additive action, the inventors of the presentinvention verified that a good effect which is insufficient in the caseof the Cu monolayer wires can be accomplished if it is applied to themultilayer wires having the core member mainly composed of Cu. That is,if the core member is formed of a Cu alloy containing B, Pd, Bi, and Ptherein within a range from 5 to 300 ppm, the skin layer is mainlycomposed of at least one kind of followings: Pd; Pt; and Ru, and thepercentage of the <111> orientations in the crystalline orientations<hkl> in the lengthwise direction at the crystal face of the surface ofthe skin layer is greater than or equal to 50%, an effect of improvingthe linearity at a long span is further enhanced. This is because thelinearity may be improved due to the synergic effect by the skin layerthat the crystalline orientations are controlled and the core membercontaining the alloy elements.

If a bonding wire with a multilayer structure employs a composition inwhich the percentage of the <111> orientations in the crystallineorientations <hkl> in the lengthwise direction at the crystal face ofthe surface of the skin layer is greater than or equal to 50%, the mainconstituent of the skin layer is Pd or Ag, the main constituent of thecore member is Cu, and the core member contains Pd at a concentrationwithin a range from 5 to 10000 ppm, it is easy to comprehensivelysatisfy suppression of any formation of flaws and scraping andstabilization of a loop shape and a loop height or stabilization ofcompressive bonding shape of a ball bonded part. In a thermal treatmentprocess in manufacturing of wires, because of a synergic action ofmaking a change in the Pd concentration uniform and moderate as Pd inthe core member and Pd, Ag in the skin layer interdiffuse one another inthe vicinity of the interface between the core member and the skinlayer, a good action of reducing peeling and scraping of the proximityof a top surface of a loop and an action of reducing dispersion inshapes, such as fall-down of a loop and bending thereof, can beaccomplished. Such concentration change is effective to not only a wholewire but also a neck part where heat at the time of ball meltingaffects, thus effective to stabilize loop heights. According to thecombination of the Cu core member and the Pd skin layer, mixing of Cuwith Pd, Ag may become nonuniform when a ball is melted and a ball shapemay become abnormal, but as the core member contains Pd, an effect ofcausing the shape of a ball bonded part to be a true circle is enhanced.Regarding the Pd concentration contained in the core member, if it isgreater than or equal to 5 ppm, the foregoing effects are verified, andpreferably, if it is greater than or equal to 200 ppm, such an effect ofimprovement becomes more remarkable. Regarding the uppermost limit ofthe Pd concentration, if it is less than or equal to 10000 ppm, any chipdamage due to hardening of a ball can be suppressed, and preferably, ifit is less than or equal to 8000 ppm, an effect of suppressing any chipdamage is further enhanced, and it is advantageous for fine pitchconnection.

In the case of an Au alloy, if more than or equal to one kind of Be, Ca,Ni, Pd, and Pt is contained at a concentration within a range from 5 to8000 ppm, the same effects can be accomplished and it becomes easy toensure a good linearity. That is, it is desirable that the core membershould be formed of an Au alloy containing more than or equal to onekind of Be, Ca, Ni, Pd, and Pt at a concentration within a range from 5to 8000 ppm, the skin layer should be mainly composed of at least onekind of followings: Pd; Pt; and Ru, and the percentage of the <111>orientations in the crystalline orientations <hkl> in the lengthwisedirection at the crystal face of the surface of the skin layer should begreater than or equal to 50%.

According to the structure of the bonding wires with a multilayerstructure, if an intermediate metal layer composed of differentconstituent from the main constituent of the skin layer and that of thecore member is present between the skin layer and the core member, itbecomes more effective to control the crystalline orientations of theskin layer. This is because crystalline orientations in a base layer mayaffect formation of the skin layer, and it is relatively easy to controlcrystalline orientations in the intermediate metal layer formed on thecore member rather than to control crystalline orientations of the coremember. More specifically, the same face-centered cubic metal as themetal of the skin layer is desirable for the intermediate metal layer.In particular, it is more preferable that the lattice constant of themetal of the intermediate metal layer should be similar to the latticeconstant of the metal of the skin layer.

That is, it is desirable that a bonding wire with a multilayer structureshould have an intermediate metal layer which is composed of a differentconstituent from the main constituent of the skin layer and that of thecore member and which is present between the skin layer and the coremember. As an effect of adding the intermediate metal layer, a peelstrength which is a barometer of the bonding strength of a wedge bondedpart can be improved as the adhesiveness between the skin layer and thecore member is improved. A simple technique of measuring a pull strengthin the vicinity of a wedge bonded part can be used as a substitute formeasurement of the peel strength. Accordingly, the peel strength can beincreased by causing the intermediate metal layer to intervene. Theconstituent of the intermediate metal layer should be selected based onthe combination of the constituent of the skin layer and that of thecore member, the foregoing metal constituents are preferable, and inparticular, Au, Pd, or Pt is more preferable. More preferably, when thecombination of the main constituents of the skin layer and the coremember, respectively, is Pd/Cu, if the main constituent of theintermediate metal layer is Au, it is advantageous for controlling ofthe crystalline orientations of the skin layer, and the respectiveadhesiveness at interfaces among skin layer/intermediate layer/coremember are relatively good. Moreover, when the combination of the mainconstituents of the skin layer and the core member, respectively, isPd/Au, if the main constituent of the intermediate metal layer is Pt, itis advantageous for controlling of the crystalline orientations and formaking the composition of the skin layer and the film thickness thereofuniform.

When the thickness of the skin layer is within a range from 0.005 to 0.2μm, it is advantageous for controlling of the foregoing crystallineorientations of the skin layer, and it facilitates comprehensivesatisfaction of requisite characteristics, such as the bonding propertyand the loop controllability. If the thickness is greater than or equalto 0.005 μm, a sufficient effect of controlling the crystallineorientations of the skin layer can be accomplished, and if it exceeds0.2 μm, hardening of a ball because of alloying becomes remarkable, anda chip damage like cracking may be caused at the time of bonding.

Preferably, if the thickness of the skin layer is within a range from0.01 to 0.15 μm, it becomes possible to stably form a loop with adesired loop shape without reducing a speed in a complex loop control.More preferably, if it is within a range from 0.020 to 0.1 μm, thisfacilitates acquisition of a stable film quality such that theprocessing efficiency of a film formation process can be increased whilepackaging the usability of the bonding wire.

If the thickness of the intermediate metal layer is within a range from0.005 to 0.2 μm, this facilitates controlling of the crystallineorientations of the skin layer, the adhesiveness of the interface withthe core member is improved, and it becomes possible to cope withcomplex loop control. Preferably, if it is within a range from 0.01 to0.1 μm, this facilitates ensuring of the uniformity of the filmthickness and the reproducibility thereof.

An interface between the skin layer and the core member is a part wherea total of the detected concentration of the electrically-conductivemetal of the skin layer is 50 mol %. Accordingly, the skin layer of thepresent invention is a surface from a part where the total of thedetected concentration of the electrically-conductive metal of the skinlayer is 50 mol %, i.e., is a part where the total of the detectedconcentration of the electrically-conductive metal of the skin layer isgreater than or equal to 50 mol %.

It is desirable that the crystalline orientation of the presentinvention should include ones that an angular difference of thecrystalline orientation relative to the lengthwise direction of thebonding wire is within 15 degree. In general, when a crystallineorientation in a particular direction is focused, individual crystalshave angular differences in some measure, and a slight angulardifference is caused depending on experimental methods including how toprepare a sample and how to measure the crystalline orientations. Whenthe angular difference is within 15 degree, characteristics ofindividual crystalline orientations are possessed and the degree oftheir influences to various characteristics of the bonding wire can beeffectively utilized.

Regarding the texture of a surface of a thin wire with a wire diameterof μm or so, it is not well known so far, and there are few reports for,in particular, the texture of an outermost surface of thin multilayerwires. In order to precisely measure the texture of a bonding wire whichis relatively soft and is a metal wire with a small diameter, anadvanced measurement technique is requisite.

Regarding how to measure a texture, an electron back scattering pattern(hereinafter, EBSP) technique recently developed can be used because itis advantageous for narrowing down a measurement-target area into a tinypart, and for acquiring information on an outermost surface only.According to measurement of a texture through the EBSP technique, atexture of a surface or a cross section of a bonding wire can bemeasured precisely with a sufficient reproducibility even though thebonding wire is a thin object. According to this measurement technique,it is possible to measure distributions of crystalline orientations oftiny crystal grains in a submicron order and those of crystallineorientations of a wire surface highly precisely with a goodreproducibility regarding a fine texture of the bonding wire.

According to the EBSP technique, in general, when concavities andconvexities of a sample and a curved face thereof are large, it isdifficult to measure crystalline orientations highly precisely. However,if measurement conditions are made appropriate, highly-precisemeasurement and analysis become possible. More specifically, a bondingwire is fixed on a plane straightly, and a flat part in the vicinity ofthe center of the bonding wire is measured through the EBSP technique.Regarding a measurement-target area, when a size thereof in thecircumferential direction is less than or equal to 50% of the wirediameter with the center in the lengthwise direction being an axis, anda size in the lengthwise direction is less than or equal to 100 μm,measurement efficiency can be enhanced in addition to the precision.Preferably, if the size in the circumferential direction is less than orequal to 40% of the wire diameter and the size in the lengthwisedirection is less than or equal to 40 μm, a time required formeasurement can be reduced, resulting in further enhancement of themeasurement efficiency.

In order to carry out highly-precise measurement through the EBSPtechnique, because an area which can be measured at a time is limited,it is desirable that more than or equal to three areas should bemeasured and average information in consideration of dispersion shouldbe obtained. It is preferable to select measurement-target areas in sucha way that measurement-target areas do not overlap so that differentareas in the circumferential direction can be measured.

For example, when a bonding wire with a wire diameter of 25 μm issubjected to measurement, the bonding wire fixed on a plane so as not tochange a wire direction as much as possible is used, ameasurement-target area at a time is set to be a size of 8 μm in thecircumferential direction around the wire axis and a size of 30 μm inthe lengthwise direction, three areas separately from one another bygreater than or equal to 1 mm are measured, and then average informationon crystalline orientations in the wire surface can be acquired.However, the measurement-target areas, and how to select themeasurement-target area are not limited to the foregoing ones, and it isdesirable to make those appropriate in consideration of a measurementdevice, a wire condition and the like.

When the crystalline orientations of the core member are subjected tomeasurement, measurement can be carried out through both of a verticalcross section in the lengthwise direction of the wire or a parallelcross section in the vicinity of the center of the wire parallel to thelengthwise direction. Preferably, a polished surface to be acquired canbe easily obtained through the vertical cross section. When a crosssection is formed by mechanical polishing, it is desirable to remove askin by etching in order to reduce a residual strain on the polishedsurface.

In analysis of the measurement result through the DBSP technique, whenan analysis software installed in the device is used, the foregoing arearatio of the area of a crystal grain in each orientation relative to themeasured area of the wire surface or a ratio of each crystallineorientation with a total area of crystal grains or a total area wherecrystalline orientations can be identified in the measured areas beingas a parent population can be calculated. A minimum unit for calculatingan area of a crystalline orientation may be a crystal grain or a tinyarea in a part of a crystal grain. Regarding a size of the crystalgrain, an average size or the like can be calculated in the lengthwisedirection and in the circumferential direction.

In manufacturing the bonding wire of the present invention, a step offorming the skin layer on the surface of the core member, and aworking/thermal treatment step of controlling the structures of the skinlayer, the diffusion layer, and the core member, and the like,respectively, are requisite.

Examples of how to form the skin layer on the surface of the core memberare plating, vapor deposition, melting, and the like. In plating,electrolytic plating and nonelectrolytic plating can be appliedseparately. Electrolytic plating has a fast plating speed, and canacquire a good adhesiveness with a base. Electrolytic plating can beone-time process, but can be separated into thin film plating so-calledflash plating, and actual plating for making a film grown thereafter,and those can be carried out through plural steps, thereby making thefilm quality stable. A solution used for nonelectrolytic plating can beclassified into a substitutional type and a reduced type, when a film tobe formed is thin, merely substitutional plating is sufficient, but whena film to be formed is thick, it is effective to perform reduced platingafter substitutional plating step by step. Nonelectrolytic techniqueneeds a simple device, is easy to carry out, but requires a more timethan electrolytic technique.

In vapor deposition, physical adsorption, such as sputtering, ionplating or vacuum deposition, or chemical adsorption like plasma CVD canbe applied. Those are all dry systems, do not need rinsing after filmformation, and thus having no possibility of surface contamination byrinsing.

Regarding at a step of performing plating or vapor deposition, both of atechnique of forming a film of an electrically-conductive metal at atarget wire diameter and a technique of once forming a film on the coremember with a large diameter and of drawing such wire several times toacquire a target wire diameter are effective. In the case of filmformation of the former technique at a final diameter, manufacturing,quality management and the like are easy, and in the case of the lattercombination of film formation and wire drawing, it is advantageous forimprovement of the adhesiveness between the film and the core member.Specific examples of individual formation techniques are a technique offorming a film on a thin wire with a target wire diameter whilesuccessively sweeping the wire in an electrolytic plating solution, anda technique of forming a film by soaking a thick wire in an electrolyticor nonelectrolytic plating solution and of drawing the wire to acquire afinal wire diameter.

The final plating technique of forming the skin layer at the final wirediameter requires only a thermal treatment process after film formation.Moreover, the thick-diameter plating technique of forming a film on athick core member requires a combination of a working process ofacquiring a target wire diameter and a thermal treatment process.

In a working process after the skin layer is formed, rolling, swaging,dice wire drawing and the like are applied selectively and separately inaccordance with purposes. Controlling a work texture, dislocation,defects of a crystalline interface and the like based on a work speed, apressurization rate, a reduction of dice area and the like affects thetexture of the skin layer and the adhesiveness thereof.

If a wire is simply subjected to film formation, working and heating,crystalline orientations in a texture at the surface of the skin layerand at the interior thereof cannot be controlled. When aprocessing-strain elimination annealing at a final wire diameter whichis applied in normal wire manufacturing processes is directly applied,loop control may become unstable because of reduction of theadhesiveness between the skin layer and the core member, and controllingof the uniformity of the skin layer in the wire lengthwise direction,the distribution of the skin layer and that of a diffusion layer at awire cross section may become difficult. Accordingly, it becomespossible to stably control the texture of the skin layer bycomprehensively and appropriately combining working conditions, such asa formation condition of the skin layer, reduction of area in a wiredrawing process and a speed, a timing of a thermal treatment process, atemperature, a speed, a time and the like.

In processes of rolling and drawing of wires, a work texture is formedand in a thermal treatment process, restoration and recrystallizationprogress so that a recrystallization texture is formed, and thosetextures are mutually associated with each other to set a final textureof the skin layer and crystalline orientations thereof. In order todirect the crystalline orientations of the skin layer to the <111>orientations, utilization of a work texture is more effective. As theprocess condition of wire drawing after formation of a film is madeappropriate, the orientation percentage to the <111> orientations can beincreased. The orientation percentage to the <111> orientations by wiredrawing varies depending on wire conditions like a composition beforeworking, but in order to set the <111> orientation percentage of theskin layer to be greater than or equal to 50%, for example, it iseffective to increase the work rate greater than or equal to 80%.Preferably, when the work rate is set to be greater than or equal to95%, an effect of increasing the <111> orientation percentage across thewhole bonding wire can be enhanced.

In a thermal treatment process, it is effective to perform thermaltreatment once or plural times. The thermal treatment process can beclassified into annealing right after film formation, annealing during aprocess, and a finish annealing at a final wire diameter, and it isimportant to apply those annealing selectively and separately. Dependingon in which process stage a thermal treatment is performed, the finalskin layer, diffusion behavior at an interface between the skin layerand the core member, and the like are changed. As an example, if processannealing is performed during a process after plating, a wire is drawn,and finish annealing is performed at a final wire diameter tomanufacture a bonding wire, it is advantageous for improvement of theadhesiveness as a diffusion layer is formed at an interface between theskin layer and the core member in comparison with a process withoutintermediate annealing.

Regarding a method of thermal treatment, as thermal treatment isperformed while successively drawing a wire and a temperature gradientis set in a furnace, not setting a furnace temperature to be constantwhich is a general thermal treatment, it becomes easy to do massproduction of the bonding wire of the present invention having the skinlayer and the core member. As specific examples, a method of locallyintroducing a temperature gradient and a method of changing atemperature in a furnace can be applied. When it is desirable tosuppress any surface oxidization of the bonding wire, it is effective toperform heating while causing inactive gases, such as N2 and Ar, to flowinside the furnace.

In a melting method, a casting technique of melting either the skinlayer or the core member is applied, and as a wire is drawn at a thickdiameter of 10 to 100 mm or so after the skin layer and the core memberare connected together, there are advantages such that the productivitybecomes good, designing of the alloy constituent of the skin layer iseasy in comparison with plating and vapor deposition, and improvement ofcharacteristics, such as the strength and the bonding property, can beeasily accomplished. Specific process can be divided into a method ofcasting a melted electrically-conductive metal around the core membermanufactured beforehand to form the skin layer and a method of using ahollow cylinder of an electrically-conductive metal manufacturedbeforehand and of casting a melted metal in the hollow part to form thecore member. The latter method of casting the core member in the hollowcylinder is preferable because it facilitates stable formation of theconcentration gradient or the like of the main constituent of the coremember in the skin layer. If a tiny amount of copper is contained in theskin layer manufactured beforehand, controlling of the copperconcentration at a surface of the skin layer becomes easy. It ispossible to omit a thermal treatment for diffusing Cu in the skin layerin the melting method, but if a thermal treatment is performed foradjusting the distributions of Cu in the skin layer, furthercharacteristic improvement can be expected.

By using such a melted metal, it becomes possible to manufacture atleast either one of the core member and the skin layer by successivecasting. According to such successive casting, processes are simplifiedin comparison with the foregoing casting method, and a wire diameter canbe thinned, thereby improving the productivity.

When bonding is performed using a multilayer copper wire having the coremember mainly composed of copper, a shield gas is requisite for forminga ball, and an N₂ gas containing H₂ at a concentration within a rangefrom 1 to 10% or a pure N₂ gas is used. A mixed gas represented by 5%H₂+N₂ is recommended for the conventional monolayer copper wires. Incontrast, in the case of the multilayer copper wires, a good bondingproperty can be acquired even if an inexpensive pure N₂ gas is used, sothat a running cost can be reduced in comparison with the case in which5% H₂+N₂ that is a standard gas is used. It is desirable that the purityof the N₂ gas should be greater than or equal to 99.95%. That is, abonding method of causing arc discharging while spraying the N2 gashaving the purity of greater than or equal to 99.95% to the leading endof a wire or therearound to form a ball, and of bonding the ball isdesirable.

Moreover, if a diffusion layer is formed between the skin layer and thecore member, the adhesiveness therebetween can be improved. Thediffusion layer is an area formed as the main constituent of the coremember and that of the skin layer interdiffuse, and has concentrationgradients of such main constituents. By forming the diffusion layer, theadhesiveness between the skin layer and the core member can be improved,peeling of the skin layer at the time of loop control and bonding can besuppressed, and by setting the concentration gradient, in comparisonwith a case in which the electrically-conductive metal has a uniformconcentration across the skin layer, wire deformation can be stabilizedin a control at the time of formation of a loop which is likely to beaffected by complex plastic deformation. Moreover, by increasing thepercentage of the <111> orientations of the surface of the skin layer tobe greater than or equal to 50%, it is verified that an effect ofsuppressing any formation of flaws and scraping is further improved ifthere is the diffusion layer having the concentration gradient.Regarding the concentration gradient in the diffusion layer, it isdesirable that the level of concentration change in the depth directionshould be greater than or equal to 10 mol % per 1 μm. Preferably, if itis greater than or equal to 5 mol % per 0.1 μm, an effect of mutuallyutilizing physicality of the skin layer and that of the core memberdifferent from each other can be anticipated without deterioratingthose. It is preferable that the thickness of the diffusion layer shouldbe within a range from 0.002 to 0.2 μm. This is because if the thicknessof the diffusion layer is less than 0.002 μm, the foregoing effect isinsufficient and is difficult to identify through analysis, and if itexceeds 0.2 μm, there is an undesirable effect to the texture of theskin layer which makes stable formation of the crystalline orientationsdifficult. Application of a thermal treatment is effective to controlthe diffusion layer. As explained above, by controlling the degree ofprogression of diffusion through a combination of a thermal treatmentand working, it becomes possible to form the desired diffusion layeruniformly in the circumferential direction of a wire or the lengthwisedirection thereof.

Regarding the concentration analysis of the skin layer, and that of thecore member and the like, a technique of performing analysis whiledigging down in the depth direction from the surface of the bonding wireby sputtering or the like, or a method of performing line analysis orpoint analysis at a wire cross section is effective. The former iseffective when the skin layer is thin, but takes a too much time formeasurement if the skin layer is thick. The latter cross-sectionalanalysis is effective when the skin layer is thick, and has an advantagethat checking of the concentration distribution across the entire crosssection and of the reproducibility at several portions is relativelyeasy, but the precision thereof decreases when the skin layer is thin.It is possible to measure the concentration by polishing a bonding wireobliquely and by increasing the thickness of the diffusion layer. Theline analysis is relatively easy for a cross section, but when it isdesirable to improve the precision of analysis, it is effective tonarrow analysis intervals of line analysis, and to perform pointanalysis while focusing on only an area which is desirable to observe inthe vicinity of an interface. As an analysis device for suchconcentration analysis, an electron probe micro analyzer (EPMA)technique, an energy dispersive x-ray spectroscopy (EDX) technique, anauger electron spectroscopy (AES) technique, a transmission electronmicroscope (TEM) and the like can be applied. In particular, accordingto the AES technique, because the spatial resolution is high, it iseffective for concentration analysis of a thin area of the outermostsurface. Moreover, for checking of an average composition or the like,it is possible to dissolve a bonding wire from the outermost surfacestep by step in an acid, and to acquire the composition of the dissolvedpart from a concentration contained in the solution. According to thepresent invention, it is not necessary that the concentration acquiredthrough all of the foregoing analysis techniques satisfies thestipulated range of the present invention, but the effect of the presentinvention can be accomplished if the concentration acquired through anyone of the foregoing analysis techniques satisfies the stipulated rangeof the present invention.

Examples

An explanation will be given of examples of the present invention.

As raw materials for bonding wires, Cu, Au, and Ag which were to be usedfor a core member and which had a high purity of greater than or equalto approximately 99.99% by weight were prepared, and Au, Pt, Pd, Ru, andRh which had a purity greater than or equal to 99.99% by weight andwhich were to be used for a skin layer or an intermediate metal layerwere also prepared.

Wires thinned to a certain wire diameter were used as a core member, andelectrolytic plating, nonelectrolytic plating, vapor deposition, meltingand the like were performed thereon and a thermal treatment wasperformed to form a different metal layer on a wire surface of the coremember. A method of forming a skin layer at a final wire diameter and amethod of forming a skin layer at a certain wire diameter and ofthinning a wire to a final wire diameter by wire drawing were adopted.Commercially-available plating solutions for a semiconductor applicationwere prepared for electrolytic plating and nonelectrolytic plating, andsputtering was applied as vapor deposition. Wires having a diameter ofapproximately 15 to 1500 μm were prepared beforehand, the wire surfacethereof was coated by vapor deposition, plating and the like, the wireswere drawn to 15 to 50 μm which were a final wire diameter, and a workstrain was removed and thermal treatment was performed in such a waythat the elongation would be within a range from 5 to 15% at last. Dicedrawing was performed up to a wire diameter of 25 to 200 μm as needed,diffusion thermal treatment was performed, and then wire drawing wasperformed again. The reduction of area of the drawing dice was setwithin a range from 5 to 15% per one dice, and those dices were combinedto adjust introduction of a work strain or the like on a wire surface.The wire drawing rate was adjusted between 20 to 500 m/min.

In a case in which the melting method was applied, a method of casting amelted metal around a core member manufactured beforehand and a methodof casting a melted metal in the center of a hollow cylindermanufactured beforehand were applied. Thereafter, processes, such asforging, rolling, and dice drawing, and thermal treatment were performedto manufacture a wire.

Regarding thermal treatment for the wires of the present invention,wires were heated while successively drawn. A scheme of introducing atemperature gradient locally and a scheme of changing a temperature in afurnace were adopted. For example, a thermal treatment furnace which wasmodified so as to be able to control and divide a temperature inside afurnace into three stages was used. As examples of a temperaturedistribution, distributions of a high temperature, a middle temperature,and a low temperature, or a middle temperature, a high temperature, anda low temperature were acquired from a wire entrance toward a wire exit,and respective heating times were also managed. In addition to thetemperature distribution, a wire sweeping speed or the like was alsomade appropriate. In an atmosphere of the thermal treatment, inactivegases, such as N₂ and Ar, were used to suppress any oxidization. A gasflow rate was adjusted within a range from 0.0002 to 0.004 m³/min, andwas used for controlling a temperature inside the furnace. A timing ofperforming thermal treatment was separated for a case in which a skinlayer was formed after thermal treatment was applied to a wire havingundergone drawing, and for a case in which thermal treatment was appliedonce or greater than or equal to twice before a process, during aprocess, or right after the skin layer was formed.

Regarding a work level by rolling and wire drawing after the skin layerwas formed, it can be sorted out by an accumulated work rate calculatedbased on an area ratio between a wire at the time of film formation anda final wire diameter. When the work rate (%) was less than 30%, it isindicated in a table by a letter R1, when the work rate was greater thanor equal to 30% but less than 70%, it is indicated in the table by aletter R2, when the work rate was greater than or equal to 70% but lessthan 95%, it is indicated in the table by a letter R3, and when the workrate was greater than or equal to 95%, it is indicated in the table by aletter R4.

In order to control a surface texture of the skin layer, it is necessaryto make a material factor, such as a material quality, a composition, athickness or the like, and a process factor, such as a film formationcondition, a process and thermal treatment condition, or the like,appropriate. In the examples, as a scheme of increasing the <111> ratioin the lengthwise direction at a surface of the skin layer, it waseffective to increase the work rate, to make an initial film thin, andto lower the temperature of a thermal treatment. As an example, when thework rate is R2 to R4, it is relatively easy to increase the <111>ratio. In contrast, in a comparative example, it was effective todecrease the work rate and to perform thermal treatment at a hightemperature or for a long time in order to reduce the <111> ratio.

Regarding observation of a texture of a wire surface, crystallineorientations were measured through the EBSP technique in a certain areaof the skin layer of a bonding wire. Regarding preparation ofmeasurement samples, three to five bonding wires were fixed on a planein such a way that wire directions differed from one another as much aspossible. Regarding an observed area, a rectangular area including awire axis was set as a measurement-target area at a time having a sizeof 5 to 10 μm in the circumferential direction and of 10 to 50 μm in thelengthwise direction. Three to ten measurement-target areas wereselected in such a manner as to be spaced apart from one another bygreater than or equal to 0.5 mm. An interval of measurement points wasset to 0.01 to 0.2 μm.

In observation of a texture of the core member, a sample that a crosssection of a bonding wire was polished and a work strain at a surfacewas reduced by chemical etching was used, and crystalline orientationswere measured through the EBSP technique. Regarding the cross section, across section vertical to the wire lengthwise direction was mainlysubjected to measurement, but a cross section parallel to the wirelengthwise direction and running through a central axis was alsomeasured in consideration of a sample condition, a reproducibility andthe like as needed.

An exclusive software (OIM analysis or the like made by TSL) was usedfor analysis of EBSP measurement data. Crystalline orientations at ameasured area were analyzed, and the percentage of crystal grains in the<111> and <100> orientations in the measured orientations was acquired.An orientation was set with reference to the lengthwise direction of abonding wire, and ones having an angular difference within 15 degree inindividual crystalline orientations were also considered for percentageacquisition. Regarding a calculation method of the percentage of thecrystal grains, two kinds of percentages were acquired: one(hereinafter, area ratio) in each orientation calculated with a wholearea of a measured area being as a parent population; and the other(hereinafter, orientation ratio) in each orientation calculated with anarea of only crystalline orientations which were able to identify beingas a parent population with reference to a certain level of reliabilityin a measured area. In the process of the latter one of acquiring anorientation ratio, calculation was carried out while excluding a partwhere it was not possible to measure crystalline orientations or a partwhere it was possible to measure crystalline orientations but the levelof reliability for orientation analysis was low. Regarding the level ofreliability, parameters may be prepared in analysis softwares, and it isdesirable to select a criterion in accordance with a condition of asample, a purpose of analysis and the like using plural kinds ofparameters, such as Confidential Index (CI value), and Image Quality (IQvalue).

Depth analysis by AES was applied for measurement of a film thickness ata wire surface, and surface analysis and line analysis by AES, EPMA orthe like were performed for observation of distributions of elementslike concentration of crystal grain interfaces. In the depth analysis byAES, measurement was performed in the depth direction while performingsputtering with Ar ions, and a depth unit was displayed in SiO₂conversion. The concentration of electrically-conductive metals in abonding wire was measured through ICP analysis, ICP mass analysis or thelike.

For connection of bonding wires, a commercially-available automatic wirebonder was used to perform ball/wedge bonding. A ball was formed at aleading end of a wire by arc discharging, and bonded to an electrodefilm on a silicon substrate, while another end of the wire was subjectedto wedge bonding on a lead terminal. A pure N₂ gas was mainly used as ashield gas used for suppressing any oxidization at the time of ballformation. A gas flow rate was adjusted within a range from 0.001 to0.01 m³/min.

An Al-alloy film (Al-1% Si-0.5% Cu film, Al-0.5% Cu film) which was amaterial of an electrode on a silicon substrate and which had athickness of 1 μm was used as a bonding partner. Conversely, as apartner for wedge bonding, a lead frame having an Ag plating (thickness:2 to 4 μm) on a surface thereof was used. Note that regarding thebonding property to an Au/Ni/Cu electrode on a BGA substrate, it wasverified that the same effect as that of the lead frame can be acquiredusing some of the wire samples.

Regarding evaluation for flaws, scraping and the like on a wire surface,those were checked through observation of the exterior appearance of abonded loop. Evaluation could be made including any effects prior toloop formation, such as flaws, scraping and the like caused in a wiremanufacturing process. A wire diameter was set to 25 μm. Wire lengthswere set to be two kinds: a general-purpose span of 2 mm; and a longspan of 5 mm, trapezoidal loops were formed with a target height beingset to 100 to 250 μm, and 1000 bonding wires for each kind were observedthrough a projector. Observation for flaws was made around the externalside of a loops, and, for scraping, made around a neck part in thevicinity of a ball bonded part having a higher occurrence frequency, andflaws having a size of greater than or equal to 10 μm were counted. Forlow-loop evaluation, low loops having a target height set toapproximately 65 μm were formed, and occurrence of flaws and scrapingwere observed through the same fashion. In general, the longer the wirelength is, or the lower the loop height is, the more the frequency thata wire surface is scratched increases, so that evaluation becomes harsh.When there were greater than or equal to four scraping and flaws werenoticeable, it was determined that a problem was present and it isindicated in the table by a cross mark in the table, when the number ofscarping was one to three but there were lots of flaws and a negativeeffect like clogging of a capillary was concerned, it was determinedthat improvement was necessary and it is indicated in the table by atriangular mark, when the number of scraping was one to three and therewas no big flaw acknowledged as a problem, it was determined that a wiresurface was relatively good and it is indicated in the table by acircular mark, and when there was no scraping and no noticeable flaw, itwas determined as stable and it is indicated in the table by a doublecircular mark. Determination of flaws and scraping might be affected bya personal judgment by an observer in some levels, so that evaluationwas made by greater than or equal to two observers, and ranking was madewith average information.

For evaluation of flaws and scraping on a wire surface in the case of athin wire, wires with two kinds of wire diameters: 22 μm; and 18 μm wereprepared. A wire length was set to 2 mm, and trapezoidal loops having atarget height set to 70 to 200 μm were formed, and 1000 bonding wiresfor each kind were observed through a projector. The same criteria wereadopted for evaluation of flaws and scraping.

In order to evaluate the linearity of a bonded loop, bonding was carriedout at three kinds of wire intervals (spans): a normal span of 2 mm; along span of 5 mm; and a super long span of 7 mm. A wire diameter wasset to 25 μm. Thirty bonding wires were observed from above through aprojector, and a displacement at a part where a bonding wire was spacedapart at a maximum relative to a straight line interconnecting a ballbonded part and a wedge bonded part together was measured as an amountof curvature. When an average of such amount of curvature was less thana wire diameter of one wire, it was determined as good and it isindicated in the table by a double circular mark, when such an averagewas greater than or equal to wire diameters of two wires, it wasdetermined as bad, and it is indicated in the table by a triangularmark, and when such an average was therebetween, it would not become aproblem in general, and it is indicated in the table by a circular mark.

Regarding the stability of a loop shape in a bonding process, thirtytrapezoidal loops were connected at a long span with a wire length of 5mm so as to obtain a loop height of 200 to 250 μm, and such stabilitywas evaluated based on a standard deviation of heights. A wire diameterwas set to 25 μm. An optical microscope was used for measuring heights,and measurement was carried out at two locations in the vicinity of thehighest part of a loop and at the center of the loop. When the standarddeviation of loop heights was greater than or equal to ½ of the wirediameter, it was determined that dispersion was large, and when suchstandard deviation was less than ½, it was determined that dispersionwas small and good. Determination was made based on those criteria, whendispersion was small at three locations, it was determined that a loopshape was stable and it is indicated in the table by a double circularmark in the table, when the number of locations where dispersion waslarge was one, it was determined as relatively good and it is indicatedin the table by a circular mark, when such number was two, it isindicated in the table by a triangular mark, and when dispersion waslarge at all of the three locations, it is indicated in the table by across mark.

In determination of a bonding shape of a compressively-bonded ball part,200 bonded balls were observed, and the roundness, abnormal deformationfailure, and size precision of a shape were evaluated. A wire diameterwas set to 20 μm. Evaluation was made for two kinds of ball sizes: onein which a ball with a normal size that the ratio between an initialball and a wire diameter was 1.9 to 2.2; and the other in which a ballwith a smaller diameter that the ratio was 1.5 to 1.7. When the numberof defective ball shapes like anisotropic or petal-like shape out ofround was greater than or equal to five, it was determined as a failureand it is indicated in the table by a cross mark, when the number ofdefective ball shapes out of round was two to four, this case wascategorized into two kinds as follow. When the number of abnormaldeformation caused was greater than or equal to one, it was determinedthat an improvement was requisite for mass production and it isindicated in the table by a blacked triangular mark, and when there wasno abnormal deformation, it was determined as usable and it is indicatedin the table by a whitened triangular mark. When the number of defectiveball shapes was less than or equal to one, it was determined as good andit is indicated in the table by a circular mark.

A pull test for a wedge bonded part was carried out to evaluate a peelbonding strength. A wire diameter and a span were set to 25 μm, and 2mm, respectively. A hook grabbing a loop was moved upwardly at alocation near a wedge bonded part more than ¾ of a wire length, and thebreaking strength of a bonding wire was measured. The pull strengthvaries depending on the wire diameter of the bonding wire, a loop shape,a bonding condition and the like, a relative ratio (Rp) between the pullstrength and a wire tensile strength was acquired instead of an absolutevalue. When Rp was greater than or equal to 20%, it was determined thatthe wedge bonding property was good and it is indicated in the table bya double circular mark, when greater than or equal to 15% and less than20%, it was determined as no problem and it is indicated in the table bya circular mark, when greater than or equal to 10% and less than 15%, itwas determined that a problem might occur and it is indicated in thetable by a triangular mark, and when greater than or equal to 10%, therewould be a problem in a mass-production process and it is indicated inthe table by a cross mark.

In order to evaluate adhesiveness between the skin layer and the coremember at the time of loop formation, loops were observed from aboveusing an optical microscope, and occurrence of peeling of the skin layerwas checked. Using normal loops having a wire diameter of 25 μm at a3-mm span, 400 loops were observed. The number of peeling was subjectedto comparison, when such number was zero, it was determined as good andit is indicated in the table by a circular mark, when such number wasone to four, it was determined as no problem in general but improvementwould become requisite in some cases, so that it is indicated in thetable by a triangular mark, and when such number was greater than orequal to five, there would be a problem in a mass-production process,and it is indicated in the table by a cross mark.

In depth analysis through AES analysis, a diffusion layer having aconcentration gradient between the skin layer and the core member wasconfirmed, and when the thickness of the diffusion layer was within arange from 0.002 to 0.2 μm, a circular mark was put on a field of“diffusion layer” in table 1.

Regarding evaluation for any chip damage, a ball was bonded onto anelectrode film, the electrode film was removed by etching, and anydamage to an insulation film or a silicon chip was observed using anSEM. The number of electrodes observed was 400. When there was nodamage, it is indicated in the table by a circular mark, when the numberof cracks was less than or equal to two, it was determined as no problemand it is indicated in the table by a triangular mark, and when thenumber of cracks was greater than or equal to three, it was determinedas a level requiring a concern, and it is indicated in the table by across mark.

Table 1 and table 2 show examples of the bonding wire of the presentinvention and comparative examples.

TABLE 1 Aspect ratio of Percentage crystal grain in ManufacturingCrystalline orientation in lengthwise direction of wire surface methodin surface of skin layer [111] + [100] between Intermediate (A:nonelectrolytic Area ratio of in cross lengthwise Skin layer Core membermetal layer B: electrolytic Percentage relative to whole [111]orientations section of direction and Film Additive Film C: vaporcrystalline orientations (%) in wire surface core circumferential Mainthickness/ Main element Diffusion Main thickness/ deposition Work [111][100] [111] + [100] area (%) member (%) direction constituent μmconstituent mass · ppm layer constituent μm D: melting) rate Example 151 4 55 32 35 3.5 Pd 0.02 Cu — ◯ — — A R2 2 60 12 72 33 43 5.2 Pd 0.04Cu P80 ◯ — — B R2 3 73 18 91 45 68 11.3 Pd 0.03 Cu — ◯ — — B R4 4 82 587 57 32 22.4 Pd 0.07 Cu — ◯ — — B R3 5 94 0.5 94.5 82 47 27.3 Pd 0.04Cu — ◯ — — B R4 6 55 12 67 15 37 3.8 Pd 0.06 Cu — ◯ — — A R2 7 52 32 8426 64 5.6 Pd 0.08 Cu — ◯ — — B R2 8 63 19 82 28 15 7.3 Pd 0.03 Cu Pd250◯ — — B R3 9 72 15 87 35 27 4.7 Pd 0.18 Cu B30 ◯ — — D R4 10 58 25 83 3765 1.3 Pd 0.007 Cu — X — — B R1 11 73 18 91 43 82 2.7 Rh 0.01 Cu — ◯ — —B R4 12 52 23 75 35 42 3.8 Pt 0.04 Cu Zr20 ◯ — — C R2 13 65 12 77 36 396.7 Pt 0.08 Cu — ◯ — — A R3 14 77 8 85 44 24 14.0 Pt 0.15 Cu — ◯ — — BR4 15 52 3 55 37 32 2.3 Ru 0.01 Cu — ◯ — — B R1 16 65 23 88 48 48 8.2 Ru0.1 Cu — ◯ — — B R2 17 53 4 57 30 47 3.3 Pd 0.06 Au Ca30 ◯ — — A R2 1862 17 79 43 25 1.3 Pd 0.01 Au — ◯ — — D R3 19 65 3 68 38 61 4.7 Pd 0.04Au Be20 ◯ — — B R2 20 78 10 88 53 72 8.3 Pd 0.18 Au Pd5000 ◯ — — B R4 2192 1 93 72 95 28.2 Pd 0.09 Au — ◯ — — B R4 22 55 23 78 25 43 5.3 Pt 0.03Au Ni50 ◯ — — B R2 23 72 12 84 45 55 8.5 Pt 0.07 Ag — ◯ — — B R3 24 6315 78 38 63 10.2 Ru 0.03 Au — ◯ — — B R3 25 52 10 62 32 44 2.4 Pd 0.04Cu — ◯ Au 0.01 B R2 26 88 4 92 46 50 12.1 Pd 0.16 Cu — ◯ Au 0.12 B R4 2762 13 75 30 38 7.3 Ru 0.06 Cu P120 ◯ Pd 0.01 B R3 28 55 8 63 35 45 3.3Pd 0.06 Au — ◯ Pt 0.03 B R2 29 73 15 88 63 72 12.2 Pt 0.15 Au — ◯ Pd0.09 B R3 30 54 32 86 48 55 6.1 Ru 0.02 Au Ca8 ◯ Pd 0.01 B R2 31 68 1886 43 13 6.5 Pd 0.1 Cu — ◯ — — B R3 32 57 18 75 38 10 4.3 Pt 0.03 Au — X— — B R4 33 58 20 78 42 40 3.7 Pt 0.22 Cu B10 ◯ — — B R1 34 55 18 73 4332 3.5 Pd 0.12 Cu Pd100 ◯ — — B R2 35 82 13 95 62 61 7 Pd 0.08 Cu Pd700◯ — — B R4 36 67 15 82 45 33 13 Pd 0.05 Cu Pd8000 ◯ — — B R2 37 61 17 7838 53 8.7 Pd 0.02 Cu Pd9800 ◯ — — B R3 38 58 20 78 35 31 6.5 Ag 0.08 Cu— ◯ — — B R4 39 65 18 83 43 51 4.6 Ag 0.02 Cu Pd2000 ◯ — — B R3Comparative 1 42 3 45 31 35 4.5 Pt 0.002 Cu — X — — C R1 example 2 31 536 8 25 1.5 Pd 0.003 Cu — X — — B R1 3 20 12 32 15 23 3.3 Pd 0.05 Cu — ◯— — A R1 4 18 25 43 26 40 6.2 Ru 0.02 Cu — ◯ — — B R1 5 32 12 44 19 232.1 Pd 0.23 Au — ◯ — — B R2 6 23 18 41 27 65 10.5 Pt 0.01 Au — ◯ — — BR1

TABLE 2 Evaluation Evaluation Loop height Ball bonding Peel Peeling forflaw and for flaw and Linearity of loop stability shape (20 μmφ) bondingof skin scraping (25 μmφ) scraping with thin (25 μmφ) (25 μmφ) Small-strength layer Span Span Low wire (Span 2 mm) Span Span Span Span SpanNormal diameter (25 μmφ, above Chip 3 mm 5 mm loop 20 μmφ 18 μmφ 3 mm 5mm 7 mm 3 mm 5 mm ball ball span 3 mm) loop damage Example 1 ⊚ ◯ Δ ⊚ ◯ ⊚◯ Δ ◯ Δ ⊚ ◯ ◯ ◯ ◯ 2 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ◯ ⊚ ◯ ◯ ◯ ◯ 3 ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚⊚ ⊚ ◯ ◯ ◯ 4 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ ◯ 5 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯◯ 6 ⊚ ◯ Δ ◯ Δ ⊚ ◯ Δ ⊚ ◯ ⊚ ◯ ◯ ◯ ◯ 7 ⊚ ◯ Δ ◯ Δ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ 8 ⊚ ⊚◯ ◯ Δ ⊚ ⊚ Δ ⊚ ⊚ ◯ Δ ◯ ⊚ ◯ 9 ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ Δ ⊚ ⊚ ◯ Δ ◯ ◯ ◯ 10 ⊚ ◯ Δ ⊚ ◯ ◯Δ Δ ⊚ ⊚ ⊚ ⊚ ◯ Δ ◯ 11 ⊚ ⊚ ⊚ ⊚ ◯ ◯ Δ Δ ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ 12 ⊚ ◯ Δ ⊚ ◯ ⊚ ⊚ Δ ⊚◯ ⊚ ◯ ◯ ◯ ◯ 13 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ◯ ⊚ ◯ ◯ ◯ ◯ 14 ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ Δ◯ ◯ ◯ 15 ⊚ ◯ Δ ⊚ ◯ ◯ Δ Δ ◯ Δ ⊚ ◯ ◯ ◯ ◯ 16 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ◯ ◯ ◯ ◯17 ⊚ ◯ Δ ⊚ ◯ ⊚ ⊚ Δ ⊚ ◯ ⊚ ◯ ◯ ◯ ◯ 18 ⊚ ⊚ ◯ ⊚ ◯ ◯ Δ Δ ⊚ ◯ ◯ Δ ◯ ◯ ◯ 19 ⊚ ⊚◯ ⊚ ◯ ⊚ ⊚ Δ ⊚ ◯ ⊚ ⊚ ◯ ◯ ◯ 20 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ 21 ⊚ ⊚ ⊚ ⊚ ⊚⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ 22 ⊚ ◯ Δ ◯ Δ ⊚ ⊚ ◯ ⊚ ◯ ⊚ ◯ ◯ ◯ ◯ 23 ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ◯⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ 24 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ◯ ◯ ◯ 25 ⊚ ◯ Δ ⊚ ◯ ◯ Δ Δ ⊚ ◯ ⊚◯ ⊚ ◯ ◯ 26 ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ◯ 27 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ◯ ⊚ ◯ ⊚ ◯◯ 28 ⊚ ◯ Δ ⊚ ◯ ⊚ ◯ Δ ⊚ ◯ ⊚ ◯ ⊚ ◯ ◯ 29 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ◯ 30 ⊚◯ Δ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ◯ 31 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ⊚ ▴ ▴ ◯ ◯ ◯ 32 ⊚ ◯ Δ ⊚◯ ⊚ ◯ Δ ⊚ ◯ ▴ ▴ ◯ Δ ◯ 33 ⊚ ◯ Δ ⊚ ◯ ⊚ ◯ Δ ⊚ ◯ ⊚ ◯ ◯ ◯ Δ 34 ⊚ ◯ Δ ⊚ ◯ ⊚ ◯◯ ⊚ ◯ ⊚ ◯ ◯ ◯ ◯ 35 ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ◯ 36 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ⊚ ⊚ ⊚⊚ ◯ ◯ ⊚ ◯ 37 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ◯ ⊚ ⊚ ◯ ⊚ Δ 38 ⊚ ◯ Δ ⊚ ◯ ⊚ ⊚ ◯ ⊚ ◯ ⊚ ◯ ◯◯ ◯ 39 ⊚ ⊚ ◯ ⊚ ◯ ⊚ ◯ Δ ⊚ ⊚ ⊚ ⊚ ◯ ⊚ ◯ Comparative 1 Δ X X X X ◯ ◯ X X X ◯Δ Δ Δ ◯ Example 2 X X X X X X X X X X Δ X Δ Δ ◯ 3 X X X X X X X X X X ΔX Δ ◯ ◯ 4 X X X X X ◯ ◯ X X X ◯ Δ Δ ◯ ◯ 5 X X X X X X X X X X Δ X Δ ◯ Δ6 X X X X X ⊚ ◯ Δ X X ◯ Δ Δ ◯ ◯

The bonding wire according to claim 1 corresponds to examples 1 to 39,the bonding wire according to claim 2 corresponds to examples 2 to 14,16, and 18 to 39, the bonding wire according to claim 3 corresponds toexamples 1 to 30, and 33 to 39, the bonding wire according to claim 4corresponds to examples 1 to 7, 10 to 13, 15 to 17, 19 to 30, and 33 to39, the bonding wire according to claim 5 corresponds to examples 1 to9, 12 to 14, 16, 17, 19 to 24, and 26 to 39, the bonding wire accordingto claim 6 corresponds to examples 1 to 5, 9 to 21, and 23 to 39, thebonding wire according to claim 7 corresponds to examples 1 to 10, and12 to 39, the bonding wire according to claim 8 corresponds to examples1 to 22, and 24 to 39, the bonding wire according to claim 9 correspondsto examples 25 to 30, the bonding wire according to claim 10 correspondsto examples 1 to 32, and 34 to 39, the bonding wire according to claim11 corresponds to examples 1 to 9, 11 to 31, and 33 to 39, the bondingwire according to claim 12 corresponds to examples 2, 8, 9, 12, 27, 33,and 34, the bonding wire according to claim 13 corresponds to examples8, 34 to 37, and 39, and the bonding wire according to claim 14corresponds to examples 17, 19, 20, 22, and 30. Comparative examples 1to 6 represent results which do not satisfy the condition of claim 1.

FIG. 1 shows an example of an EBSP measurement result at a surface ofthe bonding wire of the example 4. An area where crystallineorientations in the wire lengthwise direction are within 15 degree of anangular difference from the <111> orientation is colored, and acrystalline interface having an angular difference greater than or equalto 15 degree is indicated by a line. The <111> area ratio in FIG. 1 is88%.

Some of evaluation results for representative examples of individualclaims will now be explained.

According to the bonding wires with a multilayer structure of theexamples 1 to 39, it is verified that flaws and scraping on a wiresurface are reduced because the percentage of <111> (<111> orientationpercentage) in crystalline orientations in the surface of the skin layerwas greater than or equal to 50% according to the present invention. Incontrast, according to the comparative examples 1 to 6 relating tobonding wires with a multilayer structure in which the <111> orientationpercentage in the surface of the skin layer was less than 50%, a largenumber of scraping and flaws are observed in normal loop formation. Aspreferable examples, according to the examples 2 to 5, 8, 9, 11, 13, 14,16, 18 to 21, 23, 24, 26, 27, 29, 31, 35 to 37, and 39 in which the<111> orientation percentage in the skin layer was greater than or equalto 60%, flaws and scraping are reduced at a long span, and according tothe examples 3 to 5, 9, 11, 14, 20, 21, 23, 26, 29, and 35 in which the<111> orientation percentage is greater than or equal to 70%, it isverified that failures, such as flaws and scraping, are suppressed undera sever condition with low looping.

According to the bonding wires with a multilayer structure of theexamples 2 to 14, 16, 18 to 39, it is verified that dispersion in loopheights are suppressed and stabilized under a normal looping conditionat a 3-mm span because the total orientation percentage of the <111>orientation and the <100> orientation in the surface of the skin layerwas greater than or equal to 60% according to the present invention.Preferably, according to the examples 3 to 5, 7 to 11, 14, 16, 20, 21,23, 26, 29, 30, 31, 35, 36, and 39 in which such orientation percentagewas greater than or equal to 80%, a loop height can be stabilized at along span of 5 mm.

According to the bonding wires with a multilayer structure of theexamples 1 to 7, 10 to 13, 15 to 17, 19 to 30, and 33 to 39, it isverified that petal-like deformation failure is reduced in a conditionwith a normal ball size and the shape of a ball is stabilized becausethe total orientation percentage of the <111> orientation and the <100>orientation at the cross section of the core member was greater than orequal to 30% according to the present invention. Preferably, accordingto the examples 3, 7, 10, 11, 19 to 21, 23, 24, 26, 29, 30, 35, 37, and39 in which such orientation percentage was greater than or equal to50%, it is verified that the roundness of a ball bonded part is improvedeven under a sever bonding condition like small-diameter ball bonding.

According to the bonding wires with a multilayer structure of theexamples 1 to 9, 12 to 14, 16, 17, 19 to 24, and 26 to 39, it isverified that the linearity of a loop is good in a normal condition at a3-mm span because the aspect ratio between the average size of crystalgrains in the surface of the skin layer in the lengthwise direction andthat in the circumferential direction was greater than or equal to threeaccording to the present invention. Preferably, according to theexamples 2 to 5, 7, 8, 13, 14, 16, 20 to 24, 26, 27, 29, 30, 31, and 35to 38 in which such aspect ratio was greater than or equal to five, itis verified that the linearity is improved under a sever bondingcondition at a long span of 5 mm. More preferably, according to theexamples 3 to 5, 14, 21, 24, 26, 29, and 36 in which such aspect ratiowas greater than or equal to ten, it is verified that the linearity isimproved at a super long span of 7 mm which is a sever loopingcondition.

According to the bonding wires with a multilayer structure of theexamples 1 to 5, 9 to 21, and 23 to 39, it is verified that flaws andscraping on a wire surface are reduced in thin wires having a wirediameter of 22 μm because the ratio of the area (<111> area ratio) ofcrystal grains where crystalline orientations in the wire lengthwisedirection in the surface of the skin layer were <111> relative to thewire surface was greater than or equal to 30% according to the presentinvention. Preferably, according to the examples 3 to 5, 11, 14, 16, 18,20, 21, 23, 26, 29, 30, 31, 33 to 36, and 39 in which such <111> arearatio was greater than or equal to 40%, it is verified that flaws andscraping are suppressed even in the case of an extremely thin wire witha further smaller diameter of 18 μm. More preferably, according to theexamples 4, 5, 20, 21, 29, and 35 in which such <111> area ratio wasgreater than or equal to 50%, it is verified that an effect ofsuppressing any formation of flaws and scraping is further enhanced.

According to the bonding wires with a multilayer structure of theexamples 25 to 30, it is verified that a peel strength at a wedge bondedpart is enhanced because the <111> orientation percentage was greaterthan or equal to 50% and the intermediate metal layer was formed betweenthe skin layer and the core member.

According to the bonding wires with a multilayer structure of theexamples 1 to 30, and 33 to 39, it is verified that abnormal deformationof a ball bonded part is reduced, and the shape thereof is stabilized ata normal ball size because the total orientation percentage of the <111>orientation and the <100> orientation at the cross section of the coremember was greater than or equal to 15% according to the presentinvention.

According to the bonding wires with a multilayer structure of theexamples 1 to 32, and 34 to 39, it is verified that any chip damage isreduced and good because the thickness of the skin layer was within arange from 0.005 to 0.2 μm according to the present invention. As acomparative example, according to the example 33, it is verified thatchip damage increases because the thickness of the skin layer exceeded0.2 μm.

According to the bonding wires with a multilayer structure of theexamples 1 to 9, 11 to 31, and 33 to 39, it is verified that there is nopeeling above a loop and the adhesiveness of the skin layer is goodbecause a diffusion layer having a concentration gradient was formedbetween the skin layer and the core member according to the presentinvention.

According to the bonding wires with a multilayer structure of theexamples 2, 8, 9, 12, 27, 33, and 34, it is verified that the linearityof a loop at a span of 5 mm or so is improved because the core membermainly composed of Cu and contains greater than or equal to one kind offollowings: B; Pd; P; and Zr at a concentration within a range from 5 to300 ppm according to the present invention. Likewise, according to theexamples 17, 19, 20, 22, and 30, it is verified that the linearity isimproved because the core member mainly composed of Au and containsgreater than or equal to one kind of followings: Be; Ca; Ni; and Pdaccording to the present invention. Regarding the effect of improvingthe linearity of a loop at a 5-mm span or so, it is also effective toset the aspect ratio in lengthwise direction/circumferential directionto be greater than or equal to five as explained above, but it may bedifficult in some times to distinguish such effect from the effectacquired by addition of an alloy element. In contrast, according to theexamples 9, 12, 17, 19, and 33, it is verified that the linearity at a5-mm span or so can be improved because the foregoing alloy element wascontained even though the aspect ratio was less than five.

According to the bonding wires with a multilayer structure of theexamples 8, 34 to 37, and 39, it is verified that a good effect ofreducing any peeling and scraping in the vicinity of the top surface ofa loop is accomplished because the core member mainly composed of Cu andcontains Pd within a range from 5 to 10000 ppm, and the skin layermainly composed of Pd or Ag according to the present invention.Preferably, according to the examples 8, 35 to 37, and 39, the foregoingeffect becomes more remarkable because the Pd concentration was greaterthan or equal to 200 ppm. Moreover, according to the examples 8, 34 to36, and 39, it is verified that any chip damage is suppressed becausethe Pd content was within a range from 5 to 8000 ppm.

1. A bonding wire for semiconductor devices, the bonding wirecomprising: a core member formed of an electrically-conductive metal;and a skin layer formed on the core member and mainly composed of ametal different from the core member, and wherein the metal of the skinlayer is a face-centered cubic metal, a thickness of the skin layer iswithin a range from 0.005 to 0.09 μm, an orientation ratio of <111>orientations in crystalline orientations <hkl> in a wire lengthwisedirection at a crystal face of a surface of the skin layer is greaterthan or equal to 50%, and said <111> orientations have an angulardifference relative to the wire lengthwise direction, said angulardifference being within 15 degrees.
 2. The semiconductor-device bondingwire according to claim 1, further comprising a diffusion layer formedbetween the skin layer and the core member and having a concentrationgradient of each main constituent of the skin layer and the core member.3. The semiconductor-device bonding wire according to claim 2, wherein amain constituent of the skin layer is at least one of the followings:Pd; Pt; Ru; and Ag.
 4. The semiconductor-device bonding wire accordingto claim 3, wherein a main constituent of the core member is at leastone of the followings: Cu; and Au.
 5. The semiconductor-device bondingwire according to claim 4, wherein the main constituent of the coremember is Cu and the core member contains one or more of the followings:B; Pd; Bi; P; and Zr at a concentration within a range from 5 to 300 ppmin total.
 6. The semiconductor-device bonding wire according to claim 4,wherein the main constituent of the core member is Cu and the coremember contains Pd at a concentration within a range from 5 to 10000ppm, and the main constituent of the skin layer is Pd or Ag.
 7. Thesemiconductor-device bonding wire according to claim 4, wherein the mainconstituent of the core member is Au and the core member contains one ormore of the followings: Be; Ca; Ni; Pd and Pt at a concentration withina range from 5 to 8000 ppm in total.
 8. The semiconductor-device bondingwire according to claim 3, wherein the main constituent of the coremember is Cu and the core member contains one or more of the followings:B; Pd; Bi; P; and Zr at a concentration within a range from 5 to 300 ppmin total.
 9. The semiconductor-device bonding wire according to claim 3,wherein the main constituent of the core member is Cu and the coremember contains Pd at a concentration within a range from 5 to 10000ppm, and the main constituent of the skin layer is Pd or Ag.
 10. Thesemiconductor-device bonding wire according to claim 3, wherein the mainconstituent of the core member is Au and the core member contains one ormore of the followings: Be; Ca; Ni; Pd and Pt at a concentration withina range from 5 to 8000 ppm in total.
 11. The semiconductor-devicebonding wire according to claim 2, wherein a main constituent of thecore member is at least one of the followings: Cu; and Au.
 12. Thesemiconductor-device bonding wire according to claim 11, wherein themain constituent of the core member is Cu and the core member containsone or more of the followings: B; Pd; Bi; P; and Zr at a concentrationwithin a range from 5 to 300 ppm in total.
 13. The semiconductor-devicebonding wire according to claim 11, wherein the main constituent of thecore member is Cu and the core member contains Pd at a concentrationwithin a range from 5 to 10000 ppm, and the main constituent of the skinlayer is Pd or Ag.
 14. The semiconductor-device bonding wire accordingto claim 11, wherein the main constituent of the core member is Au andthe core member contains one or more of the followings: Be; Ca; Ni; Pdand Pt at a concentration within a range from 5 to 8000 ppm in total.15. The semiconductor-device bonding wire according to claim 1, whereina main constituent of the skin layer is at least one of the followings:Pd; Pt; Ru; and Ag.
 16. The semiconductor-device bonding wire accordingto claim 15, wherein a main constituent of the core member is at leastone of the followings: Cu; and Au.
 17. The semiconductor-device bondingwire according to claim 16, wherein the main constituent of the coremember is Cu and the core member contains one or more of the followings:B; Pd; Bi; P; and Zr at a concentration within a range from 5 to 300 ppmin total.
 18. The semiconductor-device bonding wire according to claim16, wherein the main constituent of the core member is Cu and the coremember contains Pd at a concentration within a range from 5 to 10000ppm, and the main constituent of the skin layer is Pd or Ag.
 19. Thesemiconductor-device bonding wire according to claim 16, wherein themain constituent of the core member is Au and the core member containsone or more of the followings: Be; Ca; Ni; Pd and Pt at a concentrationwithin a range from 5 to 8000 ppm in total.
 20. The semiconductor-devicebonding wire according to claim 15, wherein the main constituent of thecore member is Cu and the core member contains one or more of thefollowings: B; Pd; Bi; P; and Zr at a concentration within a range from5 to 300 ppm in total.
 21. The semiconductor-device bonding wireaccording to claim 15, wherein the main constituent of the core memberis Cu and the core member contains Pd at a concentration within a rangefrom 5 to 10000 ppm, and the main constituent of the skin layer is Pd orAg.
 22. The semiconductor-device bonding wire according to claim 15,wherein the main constituent of the core member is Au and the coremember contains one or more of the followings: Be; Ca; Ni; Pd and Pt ata concentration within a range from 5 to 8000 ppm in total.
 23. Thesemiconductor-device bonding wire according to claim 1, wherein a mainconstituent of the core member is at least one of the followings: Cu;and Au.
 24. The semiconductor-device bonding wire according to claim 23,wherein the main constituent of the core member is Cu and the coremember contains one or more of the followings: B; Pd; Bi; P; and Zr at aconcentration within a range from 5 to 300 ppm in total.
 25. Thesemiconductor-device bonding wire according to claim 23, wherein themain constituent of the core member is Cu and the core member containsPd at a concentration within a range from 5 to 10000 ppm, and the mainconstituent of the skin layer is Pd or Ag.
 26. The semiconductor-devicebonding wire according to claim 23, wherein the main constituent of thecore member is Au and the core member contains one or more of thefollowings: Be; Ca; Ni; Pd and Pt at a concentration within a range from5 to 8000 ppm in total.